Having then gifts differing according to the grace that is given to us, whether prophecy, [let us prophesy] according to the proportion of faith;
Note 2 at Ro 12:6: It must be remembered that Paul was not teaching on the function and administration of these seven gifts that he mentioned here like he taught on the nine gifts of the Spirit in 1Co 12-14. He was simply mentioning these gifts to illustrate his point that different people in the body of Christ have different positions or functions. However, there are some truths concerning these gifts that can be gleaned from these scriptures.
First, it needs to be pointed out that all believers can operate in the gifts listed here, but that doesn't mean that is their ministry. For instance, they can and should be able to teach others, but that doesn't make them teachers. Paul said "ye may all prophesy one by one" (1Co 14:31), but he also made it clear that not all are called to be prophets (1Co 12:29). It is definite that believers should all show mercy and be givers, but some people are given supernatural gifts in these areas. Paul was describing that here.
Concerning the gift of giving, Paul said that giving should be done with simplicity (see note 7 at Ro 12:8). Those that rule should be diligent about it (see note 8 at Ro 12:8), and those who have the gift of mercy should administer it with cheerfulness.
Note 3 at Ro 12:6: The Greek word that was translated "prophecy" here is "PROPHETEIA," and it "signifies the speaking forth of the mind and counsel of God" (Vine's Expository Dictionary). This originally applied to Old Testament prophets who predicted future events, but it came to be applied to any messengers who were inspired by God as they spoke. This would apply to preachers today if they are speaking under the anointing of the Holy Spirit.
This verse is saying essentially the same thing that Peter said in 1Pe 4:11. If we are going to prophesy, let's do it according to the ability that God gave us--the measure of faith (see note 16 at Ro 12:3).